In a high-voltage, direct current (HVDC) electric power transmission system, direct current (DC) is used for the bulk transmission of electrical power. DC is often preferred for transmitting electric power over long distances, as electrical losses are lower than in corresponding AC transmission systems. In addition to this, DC transmission line costs over long distances are lower. This is because DC requires smaller conductor area than AC, as there is no need to support three phases and there is no skin effect.
In HVDC, high voltage AC must be converted to high voltage DC (rectification) before transmission, and high voltage DC must be reconverted to AC afterwards (inversion). Typically, line commutated converters (LCC) or voltage source converters (VSC) are used for rectification and inversion. LCC systems are often preferred to VSC systems, as larger power can be transmitted using LCC. The maximum power of a VSC system is limited by the power handling capability of power electronic devices. Some examples of power electronic devices include IGBTs (Insulated Gate Bipolar Transistors), IGCTs (Integrated Gate-Commutated Thyristors), GTOs (Gate Turn-off Thyristors), MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors). Recently, LCC systems have been used to transmit up to 11 GW of electric power.
However, VSC systems typically employ transistors which can be switched on and off, while LCCs employ thyristors (more precisely thyristor valves) which can only be switched on. A thyristor begins conducting when it is forward biased and its gate terminal receives a current trigger, and will continue to conduct until it is no longer forward biased. Because of this, LCC systems are susceptible to commutation failure during faults on the AC side. As will be understood by the skilled reader, commutation is the process of switching conduction of the DC current from a conductor associated with one AC phase to a conductor associated with another AC phase. Commutation failure can mean that even after the fault has been cleared, the system may need to be shut down and restarted, potentially leading to blackout.
In addition to this, in LCC systems, because of the time at which the commutation starts and the duration of the commutation, the current in the converter lags the voltage, and the system consumes reactive power. This is different from VSC, which can produce or consume reactive power on demand.
The present invention has been devised with the foregoing in mind.